Conventional and ion-selective intracellular microelectrode techniques will be employed to characterize the active and passive electrical properties of the cell membranes and the paracellular pathway of Necturus gallbladder epithelium. The long term goal of this project is to describe quantitatively the properties of each of these pathways and their role in salt transport (i.e., total and partial ionic conductances, electrochemical potential gradients, location and nature of active and passive transport mechanisms). In addition to the study of the basal properties of the tissue in vitro, the experiments proposed include the study of the effects of experimental perturbations such as: changes in the ionic composition pH and/or osmolality of the bathing media, addition of drugs (cardiac glycosides, metabolic inhibitors, ionophores) and intracellular iontophoresis.